1. Field of the Invention
The present invention relates to a mesostructured thin film, and a process for producing the mesostructured thin film. In particular, the present invention relates to a mesostructured silica thin film, a mesoporous silica thin film, a process for producing the mesostructured silica thin film, and a process for producing the mesoporous silica thin film.
2. Related Background Art
Porous materials are used in various fields such as adsorption, and separation. According to IUPAC, the porous materials are classified into micorporous materials having a pore diameter of less than 2 nm, mesoporous materials having a pore diameter of 2 to 50 nm, and macroporous materials having a pore diameter of more than 50 nm. Known microporous materials include zeolites such as natural aluminosilicates and synthetic aluminosilicates, and metal phosphates. These porous materials are employed for selective adsorption, shape-selective catalytic reactions, molecular-sized reactors, and so forth by utilizing the fine pore size.
Known microporous crystalline materials have pore diameters of not more than about 1.5 nm. A solid having a larger pore diameter is demanded for adsorption and reaction of bulkier compounds not adsorbable by the micropore. As the materials having the larger pores, there are known silica gels, pillared clays, and the like. However, these materials have a broad pore size distribution, and the pore size cannot readily be controlled.
With such a background, two methods have been disclosed at about the same time for synthesizing mesoporous silica having mesopores of a uniform size arranged in a honeycomb shape. The one method synthesizes a material called MCM-41 by hydrolysis of a silicon alkoxide in the presence of a surfactant (Nature, vol. 359, p. 710). The other method synthesizes a material called FSM-16 by intercalation of an alkylammonium in interlaminar spaces of kanemite, a kind of layered polysilicate (Journal of Chemical Society, Chemical Communications, vol. 1993, p. 680). In both methods, it is considered that surfactant assembly acts as structure-directing agent of mesostructured silica. These substances are useful as a catalyst for bulky molecules which cannot enter the pores of zeolite, and are promising in application as a functional material such as optical materials and electronic materials.
In application of such a mesoporous material having a regular porous structure as a functional material other than catalysts, the technique for uniformly holding the material on a substrate is important. The method for forming a uniform mesoporous thin film on a substrate includes a spin coating method as shown in Chemical Communications, vol. 1996, p. 1149, and a dip coating method as shown in Nature, vol. 389, p. 364, and a deposition method of forming a film on the surface of a solid material by deposition as shown in Nature, vol. 379, p. 703.